A liquid crystal display (LCD) is a thin flat display apparatus which comprises a certain amount of color or monochrome pixels and is disposed before a light source or a reflecting surface. Because the liquid crystal display has low power consumption and is characterized in high display quality, small size and light weight, it is favored by peoples and becomes a main display apparatus. Nowadays, the major one of the liquid crystal display is the thin film transistor (TFT) liquid crystal display.
As the area of the TFT-LCD being enlarged, a viewing angle is increased, and therefore the contrast and the definition of the image is decreased due to a result of birefringence variation of the liquid crystal molecules in a liquid crystal layer following on variation of the viewing angle. For a normal liquid crystal display panel, the brightness is rapidly decreased (i.e. become darker) and the color is changed when the normal liquid crystal display panel is watched from a certain angle. A conventional liquid crystal display usually provides a 90 degree viewing angle, i.e. 45 degree viewing angle in the left and right side, respectively. The nematic liquid crystal is a substance having birefringence Δn. The light passing through the liquid crystal molecule is divided into an ordinary ray and an extraordinary ray. If the light is obliquely incident into the liquid crystal molecule, two refracted light would be generated. The birefringence of the light is Δn=ne−no, wherein ne represents the refractive index of the liquid crystal molecule for the ordinary ray, and no represents the refractive index of the liquid crystal molecule for the extraordinary ray. Therefore, after the light passing through the liquid crystal clamped between the top and bottom glasses, a phase retardation would be occurred on the light. The light characteristic of a liquid crystal cell is measured by the phase retardation Δn·d, which is so called as optical path difference, wherein Δn is the birefringence and d is the depth of the liquid crystal cell. Different phase retardation under different viewing angles is the reason why the problem of viewing angle occurs. A good phase retardation of an optical compensation film could cancel the phase retardation of the nematic liquid crystal, such that the visual angle of the liquid crystal panel can be increased.
The principle of compensation of the optical compensation film is to correct the phase difference generated from the liquid crystal at different viewing angles to symmetrically compensate the birefringence nature of the liquid crystal molecule. By applying the optical compensation film for compensating, light leakage of the dark image can be effectively reduced and the contrast of an image in a certain range of viewing angle can be greatly increased. In view of the function and object of the optical compensation film, the optical compensation film can be divided into a phase difference film that purely changes phases, a chromatic compensation film, a visual angle enlargement film, etc. By applying the optical compensation film, light leakage of the dark image can be effectively reduced, the contrast and chromaticity of an image in a certain range of viewing angle can be greatly improved, and a part of the gray level inversion can be overcome. The main parameters for measuring the characteristic of the optical compensation film comprises an in-plane compensation value Ro in the plane direction, a depth compensation value Rth in the depth direction, the refractive index N and the film depth D, which satisfies the following equation:Ro=(Nx−Ny)·D; Rth=[(Nx+Ny)/2−Nz]·D 
Wherein, Nx is the refractive index along a slow axis (an axis with a biggest refractive index, i.e. the vibration direction of the light with slower transmitting speed) in the plane of the film, Ny is the refractive index along a fast axis (an axis with a smallest refractive index, i.e. the vibration direction of the light with faster transmitting speed, and is perpendicular to Nx) in the plane of the film, and Nz is the refractive index in the plane direction of the film (perpendicular to Nx and Ny).
For different liquid crystal display mode, i.e. different type of liquid crystal cell, the applied optical compensation film should be different and the values of Ro and Rth should be adjusted to an adequate value. The optical compensation films used in the large scale liquid crystal TV nowadays are mostly for VA (vertical alignment) display mode. The one used in early days is the N-TAC of Konica, and then is continuously developed to the Zeonor of OPTES, the F-TAC series of Fujitsu, the X-plate of Nitto, etc.
For different optical path differences of the liquid crystal, it is necessary to design different optical compensation modes. For a liquid crystal panel with optical path difference in a range between 324.8˜361.4 nm, it can be seen from FIG. 1 and FIG. 2, wherein FIG. 1 is an isoluminance contour diagram for wide viewing angle in dark state of a conventional liquid crystal panel after being compensated by a known compensating structure, and FIG. 2 is an equal contrast ratio contour diagram in wide viewing angle of the liquid crystal panel after being compensated by the bilayer-biaxial compensating structure described above. It can be seen from FIG. 1 and FIG. 2 that there is serious light leakage at the position of which the horizontal viewing angles phi are 20˜40°, 140˜160°, 200˜220°, and 310˜330° when applying the conventional compensation structure for compensating. The viewing angles where serious light leakage in dark state are much closer to the horizontal viewing angles, and the contrast ratio and definition in these viewing angles are low. However, the relative position between the viewers and the TV determines that the area near the horizontal viewing angles are easier to be seen by the viewers, and therefore the contrast ratio and the definition at these viewing angles make most affection to the viewing effect.